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2000 VW Beetle fuel trims/ multiple misfire.

  • Paul6004
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06 Jun 2019 18:33 #30458 by Paul6004
Customer complaint is that vehicle is low on power 'and might need a new throttle body".
Fault codes 16684 "random misfire", 16685 "cyl 1 misfire", 16686 "cly 2 misfire", 16688 "cyl 4 misfire"
17536 "bank 1 lean", 16515 "O2 sensor bank 1 low". 16656 "Fuel trim ban 1 rich", 16500 "coolant temp sensor low".
Coolant temp looks good in live data, all codes cleared. Fuel trim at idle is -15% and -17% at 3000 rpm.
Oxygen sensor stays on 0.9v at idle, 0.8 at 2000 rpm, and only starts switching at 3000 rpm.
I will pull the spark plugs out next to see if perhaps there is evidence of a leaking injector.

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06 Jun 2019 18:38 #30459 by Tyler
This screams MAF issue to me. :lol: I'd start with a quick visual on the sensor to make sure there's no garbage stuck in it. I'm betting that all the codes, aside from the ECT, are related to the same issue.

While you're under the hood, give the ECT connector a wiggle while watching scan data. The sensor is a common problem. Should be stuck in the coolant manifold at the back of the cylinder head.

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06 Jun 2019 18:48 #30460 by Paul6004
Replied by Paul6004 on topic 2000 VW Beetle fuel trims/ multiple misfire.
Thanks Tyler, something I forgot to mention is that I was concerned that air flow at idle was showing at 7.5 g/s.
Surely if there was dirt on the sensor it would show less air flow though?

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06 Jun 2019 19:21 #30461 by Desmond6004
Replied by Desmond6004 on topic 2000 VW Beetle fuel trims/ multiple misfire.
I had removed two of the plugs that were accessible and in my haste to check coolant temp in live data I found that this vehicle
can start and run [noisily] on two cylinders :D

Getting involved in discussions because I have a lot to learn still.
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06 Jun 2019 19:30 #30462 by Desmond6004
Replied by Desmond6004 on topic 2000 VW Beetle fuel trims/ multiple misfire.
The MAF sensor looked clean but I gave it a spray anyway, idle air flow has gone from 7.5 to 4.8 g/s.
Fuel trims at higher revs have dropped back and the O2 sensor is now switching at lower revs.
STFT is still -25% at idle though but possibly because the engine is still fouled up. I'll order a new sensor anywa
to be safe.


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06 Jun 2019 21:07 #30468 by Desmond6004
Replied by Desmond6004 on topic 2000 VW Beetle fuel trims/ multiple misfire.
Thanks for the help - always good to have someone to steer my suspicions in the right direction :)
The 'root cause' of the problem appears to be the air hose that had fallen out of the intake which
resulted in unfiltered air going through the system.


A new MAF sensor has been fitted and now idling is at 3.5 g/s and short term fuel trims are doing battle with the long term fuel trims which
I imagine will eventually level out.


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06 Jun 2019 21:07 #30469 by guafa
Hi guys,

What engine has this car?

4.8 g/s is normal for something like 3.6 liter

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06 Jun 2019 21:15 - 06 Jun 2019 21:16 #30471 by guafa
ok, now 3.5 g/s with the new sensor, which makes me think the engine is a near 3 liter, isnĀ“t it?

what are those new trim numbers?
Last edit: 06 Jun 2019 21:16 by guafa.

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06 Jun 2019 21:53 #30472 by Paul6004
Replied by Paul6004 on topic 2000 VW Beetle fuel trims/ multiple misfire.
It's a 2l VW.
Minimum STFT was -10 on that display, sitting at 8.6 at the far right when idling.
After changing it I had two misfire codes return which may have been there from just before changing the sensor. I cleared them and took it for another drive and they never returned.
I'm just wondering why it would register too high when it's dirty?
I'm also wondering if the CAT will take a while to clean up - if it does.

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06 Jun 2019 22:35 - 06 Jun 2019 22:37 #30473 by guafa
3.5 g/s seems to me a little bit high (should be something like 2.6 g/s @ 700 rpm). Maybe because you have 805 rpm. ok, this is just my reference point from other engines/sizes.

Anyway, it does not matter if STFT and LTFT sum is near 0% (in this case -13.3% + 8.6% = 4.7%). good enough
Last edit: 06 Jun 2019 22:37 by guafa.

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06 Jun 2019 22:43 #30474 by Desmond6004
Replied by Desmond6004 on topic 2000 VW Beetle fuel trims/ multiple misfire.
Yeah I'm happy with it. I'm just wondering whether the CAT will be ok after running rich for so long.

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07 Jun 2019 20:39 #30500 by Tyler

Desmond6004 wrote: The 'root cause' of the problem appears to be the air hose that had fallen out of the intake which
resulted in unfiltered air going through the system.


Is that the hose you're referring to in the foreground? Not to doubt your fix, but that's the SAI feed hose to the pump. An issue there shouldn't have an impact on fuel control. :huh:

Anyway, nicely done on the fix. B) Why does the MAF read higher when it's dirty? Because that should cause a lean issue, right? I've seen it go both ways. :silly: I don't know MAF internals well enough to explain it.

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08 Jun 2019 00:33 #30503 by Desmond6004
Replied by Desmond6004 on topic 2000 VW Beetle fuel trims/ multiple misfire.

Tyler wrote: Is that the hose you're referring to in the foreground? Not to doubt your fix, but that's the SAI feed hose to the pump. An issue there shouldn't have an impact on fuel control. :huh:.

With that hose out unfiltered air can get sucked in through the MAF sensor.

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08 Jun 2019 08:03 #30511 by Andy.MacFadyen
Replied by Andy.MacFadyen on topic 2000 VW Beetle fuel trims/ multiple misfire.
With VW MAF sensors VW used two different suppliers on the 2 litre engines one supplied by Bosch and one supplied by Pierburgh the catch is the output voltage curves are completely different particularly at the idle speed end.

Cleaning the Bosch and Pierburgh sensors fitted to european cars usually dosen't show any improvement.

"There's always a catch ---- Catch OBD2 ."


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08 Jun 2019 20:22 #30536 by Tyler

Desmond6004 wrote: With that hose out unfiltered air can get sucked in through the MAF sensor.


Oh right, my mistake! :blush: I gotcha now.

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